Search box disappears of the main menu when the page is loaded

I´ve searched the forums but can’t find a solution. Search it is enabled. I tried unchecking then saving again, then checking it again and then re-saving, but nothing.
When the page is loaded, you can see a search box appear on the right of the main menu but then disappear again.
When want the search field and the icon without ajax.
I found on the developer tool from Chrome that disabling this options appears:

element.style {
opacity: 0;
display: none;
}

Theme version 4.1.2

Thanks a bunch
Luis